Light-scattering translucent products of polycarbonate with various light-scattering additives and mouldings produced therefrom are already known from the prior art.
Thus, for example, EP-A 634 445 discloses light-scattering compositions which comprise polymeric particles based on vinyl/acrylate having a core/shell morphology in combination with TiO2.
The use of light-scattering polycarbonate films in flat screens is described in US 2004/0066645. Polyacrylates, PMMA, polytetrafluoroethylenes, polyalkyl-trialkoxysilanes and mixtures of these components are mentioned here as light-scattering pigments.
JP 09311205 describes the use of PC/poly(4-methyl-1-pentene) blends as the matrix material for diffusers in backlight units.
JP 03078701 describes light-scattering PC sheets which contain calcium carbonate and titanium dioxide as scattering pigments and have a light transparency of approx. 40%.
JP 05257002 describes light-scattering PC sheets with scattering pigments of silica.
JP 10046022 describes PC sheets with scattering pigments of polyorganosiloxanes.
JP 08220311 describes two-layered sheets with a diffuser coextruded layer of 5 to 25 μm, which contains acrylic scattering pigments, and a base layer. The scattering pigments used here have a size of from 0.1 to 20 μm.
JP 10046018 claims a PC which contains 0.01 to 1% of crosslinked spherical polyacrylates.
JP 09011328 claims PC sheets which have a pronounced grooved structure which is applied during the extrusion.
JP 2004/029091 describes PC diffuser sheets which contain 0.3 to 20% of scattering pigment and 0.0005 to 0.1% of optical brightener.
DE-A 37 83 338 describes light-scattering polymer compositions containing a matrix polymer and particles of a core/shell polymer for the production of rear projection tv screens.
U.S. Pat. No. 6,346,311 describes light-transmissive polymer compositions containing a matrix polymer and mixtures of spheroidal and ellipsoidal particles for the production of rear projection screens.
WO 04/031285 (corresponding to DE-A 102 45 705, not pre-published) describes polymer compositions containing various optical brighteners.
However, the diffuser sheets known from the prior art have an unsatisfactory brightness, in particular in conjunction with the set of films conventionally used in a so-called backlight unit. To evaluate the suitability of the light-scattering sheets for so-called backlight units for LCD flat screens, the brightness of the total system must be considered.
In principle, a backlight unit (direct light system) has the structure described in the following. As a rule, it comprises a housing in which a varying number, depending on the size of the backlight unit, of fluorescent tubes, so-called CCFL (cold cathode fluorescent lamp), are arranged. The inside of the housing is equipped with a light-reflecting surface. The diffuser sheet, which has a thickness of 1 to 3 mm, preferably a thickness of 2 mm, lies on this illumination system. On the diffuser sheet is a set of films which can have the following functions: light scattering (diffuser films), circular polarizers, focusing of the light in the forwards direction by so-called BEF (brightness enhancing film) and linear polarizers. The linearly polarizing film lies directly under the LCD display on top.
Polycarbonate compositions in optical uses are conventionally always colour-corrected, i.e. they contain dyestuffs to adjust the colour locus in order to compensate for the slightly yellowish colour shade of the polycarbonate.
Dyestuffs which can be employed for adjustment of the colour locus in polycarbonate are in principle all dyestuffs which have a sufficiently high heat stability up to at least 300° C., so that they are not decomposed at the processing temperatures of the polycarbonate. Furthermore, the dyestuffs should not have basic functionalities, which lead to a degradation of the polymer chain of the polycarbonate.
These include dyestuffs of the following classes: anthanthrones, anthraquinones, benzimidazoles, diketopyrrolopyrroles, isoindolinols, perinones, perylenes, phthalocyanines, quinacridones and quinophthalones.
Thus e.g. MACROLEX® dyestuffs from Lanxess can be very suitably employed for colouring polycarbonate. A large number of various dyestuffs are available from this product line, e.g. the methine dyestuff MACROLEX® Yellow 6G Gran, the azo dyestuff MACROLEX® Yellow 4G, the pyrazolone dyestuff MACROLEX® Yellow 3G Gran, the quinophthalone dyestuff MACROLEX® Yellow G Gran, the perinone dyestuff MACROLEX® Orange 3G Gran, the methine dyestuff MACROLEX® Orange R Gran, the perinone dyestuffs MACROLEX® Red E2G Gran and MACROLEX® Red EG Gran and the anthraquinone dyestuffs MACROLEX® Red G Gran, MACROLEX® Red 5B Gran, MACROLEX® Red Violet R Gran, MACROLEX® Violet 3R Gran, MACROLEX® Violet B Gran, MACROLEX® Blue 3R Gran, MACROLEX® Blue RR Gran, MACROLEX® Blue 2B Gran, MACROLEX® Green 5B Gran and MACROLEX® Green G Gran.
WO 99/13007 moreover describes e.g. indigo derivatives which are suitable for dyeing polycarbonate.
DE 19747395 describes e.g. benzo(de)isoquinolinobenzo(1,2-d:4,5-d′)diimidazole-2,12-diones which can be employed as polymer-soluble dyestuffs, inter alia in polycarbonate.